A Quick Look at Additives

DN Staff

July 17, 2006

5 Min Read
A Quick Look at Additives

The focal point of design engineers when selecting advanced plastics is the type of plastic: polysulfone, nylon, polyester, or whatever. The fact is, though, that commercial grades of engineering plastics often contain five or more additives that can represent a significant percentage of the final compound.

You need to carefully consider why you need a particular additive and what impact, if any, that additives might have on the functional characteristics or the processability of the plastic compound. In general, the mechanical properties on data sheets reflect the plastic with the additive. You also need to consider the cost impact of additives, particularly if they are specially compounded for your application.

Here are the most important additives used to modify plastics
Particulate Fillers. These can be inorganic, organic, mineral, natural or synthetic. Fillers, in general, reduce the strength and the impact of the compound, but they almost always increase the stiffness to some extent. Suppliers of volume thermoplastics, such as polyethylene, often use them to reduce cost. That's typically not the case with advanced engineering resins. "We're not so focused on using fillers to reduce costs as we are for providing enhancements, like better surface appearance, conductivity and wear resistance," comments Greg Warkoski, process technology manager for Solvay Advanced Polymers, Alpharetta, GA. Materials such as wollastanite with a higher aspect ratio improve mechanical properties of the compound. Aspect ratio is the proportional relationship between width and height. (See the table on the next page for a detailed comparison of fillers and fibers). Down the road, nano-engineered materials may come into play here in a big way.

Fiber Reinforcements
The huge mainstream player for engineered compounds is glass fiber reinforcement, which allows use of semi-crystalline materials above their glass transition temperatures. (See Part One of this series "High-Temperature Plastics-Can They Really Take the Heat?" at http://rbi.ims.ca/4928-693). One of their major functions is to increase the modulus, thereby increasing the stiffness of the finished part. The concept is illustrated in the graphic on the first page of this article. Carbon or metal fibers can be used to impart strength and conductivity. "The downside of fibers is that your density is going to increase, making the parts heavier," says Warkoski. "Your modulus is going to be a lot higher, so the shape of your stress/strain curve is going to be skewed to the left." It's a particularly important consideration if you're designing a component for a snap-fit assembly where some give is important.

Addition of glass-fiber reinforcement to a semi-crystalline plastic bumps up mechanical usefulness at higher temperatures in proportion to the amount of glass added.

Processing Aids
Stabilizers are used to prevent degradation during the injection molding process, when melt streams are subjected to high temperatures and shear forces. Many compounds also contain mold release agents, which can be metallic stearates, waxes or olefinic polymers, for easier part ejection. They are used in small quantities and generally have no appreciable effect on mechanical properties.

Heat Stabilizers
Service temperature heat stabilizers (as opposed to stabilizers used during the molding process) extend the life of a plastic compound at elevated end-use temperatures. There's a lot of science in the chemistry and these are highly tailored to polymer families. Presence of a heat stabilizer increases the compound's Continuous Use Temperature or Relative Thermal Index.

Impact Modifiers
Some plastics are inherently tough, but many need additives to improve their impact resistance. A wide range of impact modifiers are available, and the choice of additive depends largely on the plastic used. Be sure you look at impact properties for the expected service temperature. Not all modifiers provide good impact at low temperature. Keep an eye on other key properties as well, since these additives can affect creep resistance, elastic modulus and elongation. Because impact modified grades are often tailored to specific end uses, it's a good idea to discuss your requirements with your material supplier.

Friction and Wear Additives
A special class of additives improves wear characteristics of plastics. These additives reduce abrasive wear when a plastic part rubs against another part. The study of friction and wear is called tribology and the important operating variables are load and speed. A common polymer used to improve tribological properties is polytetrafluoroethylene (PTFE). "If you are going to experience low load and high speed in your application, you want to go toward a PTFE," says Warkoski. High levels of PTFE in high-load applications can create creep problems. Another option is graphite powder, which also reduces coefficient of friction and is less susceptible to creep. This is a situation where the design engineer really needs to know exact operating conditions because you may encounter too much creep if you pick the wrong compound. There are other, more exotic choices, such as silicon oils and molybdenum disulfide, that are used by custom compounders.

Installment number ten in this series will focus on other additives issues to consider from a design engineer's perspective, including pigments and flame retardants.

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